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diff --git a/Documentation/filesystems/nfs/rpc-cache.rst b/Documentation/filesystems/nfs/rpc-cache.rst new file mode 100644 index 000000000000..bb164eea969b --- /dev/null +++ b/Documentation/filesystems/nfs/rpc-cache.rst @@ -0,0 +1,220 @@ +========= +RPC Cache +========= + +This document gives a brief introduction to the caching +mechanisms in the sunrpc layer that is used, in particular, +for NFS authentication. + +Caches +====== + +The caching replaces the old exports table and allows for +a wide variety of values to be caches. + +There are a number of caches that are similar in structure though +quite possibly very different in content and use. There is a corpus +of common code for managing these caches. + +Examples of caches that are likely to be needed are: + + - mapping from IP address to client name + - mapping from client name and filesystem to export options + - mapping from UID to list of GIDs, to work around NFS's limitation + of 16 gids. + - mappings between local UID/GID and remote UID/GID for sites that + do not have uniform uid assignment + - mapping from network identify to public key for crypto authentication. + +The common code handles such things as: + + - general cache lookup with correct locking + - supporting 'NEGATIVE' as well as positive entries + - allowing an EXPIRED time on cache items, and removing + items after they expire, and are no longer in-use. + - making requests to user-space to fill in cache entries + - allowing user-space to directly set entries in the cache + - delaying RPC requests that depend on as-yet incomplete + cache entries, and replaying those requests when the cache entry + is complete. + - clean out old entries as they expire. + +Creating a Cache +---------------- + +- A cache needs a datum to store. This is in the form of a + structure definition that must contain a struct cache_head + as an element, usually the first. + It will also contain a key and some content. + Each cache element is reference counted and contains + expiry and update times for use in cache management. +- A cache needs a "cache_detail" structure that + describes the cache. This stores the hash table, some + parameters for cache management, and some operations detailing how + to work with particular cache items. + + The operations are: + + struct cache_head \*alloc(void) + This simply allocates appropriate memory and returns + a pointer to the cache_detail embedded within the + structure + + void cache_put(struct kref \*) + This is called when the last reference to an item is + dropped. The pointer passed is to the 'ref' field + in the cache_head. cache_put should release any + references create by 'cache_init' and, if CACHE_VALID + is set, any references created by cache_update. + It should then release the memory allocated by + 'alloc'. + + int match(struct cache_head \*orig, struct cache_head \*new) + test if the keys in the two structures match. Return + 1 if they do, 0 if they don't. + + void init(struct cache_head \*orig, struct cache_head \*new) + Set the 'key' fields in 'new' from 'orig'. This may + include taking references to shared objects. + + void update(struct cache_head \*orig, struct cache_head \*new) + Set the 'content' fileds in 'new' from 'orig'. + + int cache_show(struct seq_file \*m, struct cache_detail \*cd, struct cache_head \*h) + Optional. Used to provide a /proc file that lists the + contents of a cache. This should show one item, + usually on just one line. + + int cache_request(struct cache_detail \*cd, struct cache_head \*h, char \*\*bpp, int \*blen) + Format a request to be send to user-space for an item + to be instantiated. \*bpp is a buffer of size \*blen. + bpp should be moved forward over the encoded message, + and \*blen should be reduced to show how much free + space remains. Return 0 on success or <0 if not + enough room or other problem. + + int cache_parse(struct cache_detail \*cd, char \*buf, int len) + A message from user space has arrived to fill out a + cache entry. It is in 'buf' of length 'len'. + cache_parse should parse this, find the item in the + cache with sunrpc_cache_lookup_rcu, and update the item + with sunrpc_cache_update. + + +- A cache needs to be registered using cache_register(). This + includes it on a list of caches that will be regularly + cleaned to discard old data. + +Using a cache +------------- + +To find a value in a cache, call sunrpc_cache_lookup_rcu passing a pointer +to the cache_head in a sample item with the 'key' fields filled in. +This will be passed to ->match to identify the target entry. If no +entry is found, a new entry will be create, added to the cache, and +marked as not containing valid data. + +The item returned is typically passed to cache_check which will check +if the data is valid, and may initiate an up-call to get fresh data. +cache_check will return -ENOENT in the entry is negative or if an up +call is needed but not possible, -EAGAIN if an upcall is pending, +or 0 if the data is valid; + +cache_check can be passed a "struct cache_req\*". This structure is +typically embedded in the actual request and can be used to create a +deferred copy of the request (struct cache_deferred_req). This is +done when the found cache item is not uptodate, but the is reason to +believe that userspace might provide information soon. When the cache +item does become valid, the deferred copy of the request will be +revisited (->revisit). It is expected that this method will +reschedule the request for processing. + +The value returned by sunrpc_cache_lookup_rcu can also be passed to +sunrpc_cache_update to set the content for the item. A second item is +passed which should hold the content. If the item found by _lookup +has valid data, then it is discarded and a new item is created. This +saves any user of an item from worrying about content changing while +it is being inspected. If the item found by _lookup does not contain +valid data, then the content is copied across and CACHE_VALID is set. + +Populating a cache +------------------ + +Each cache has a name, and when the cache is registered, a directory +with that name is created in /proc/net/rpc + +This directory contains a file called 'channel' which is a channel +for communicating between kernel and user for populating the cache. +This directory may later contain other files of interacting +with the cache. + +The 'channel' works a bit like a datagram socket. Each 'write' is +passed as a whole to the cache for parsing and interpretation. +Each cache can treat the write requests differently, but it is +expected that a message written will contain: + + - a key + - an expiry time + - a content. + +with the intention that an item in the cache with the give key +should be create or updated to have the given content, and the +expiry time should be set on that item. + +Reading from a channel is a bit more interesting. When a cache +lookup fails, or when it succeeds but finds an entry that may soon +expire, a request is lodged for that cache item to be updated by +user-space. These requests appear in the channel file. + +Successive reads will return successive requests. +If there are no more requests to return, read will return EOF, but a +select or poll for read will block waiting for another request to be +added. + +Thus a user-space helper is likely to:: + + open the channel. + select for readable + read a request + write a response + loop. + +If it dies and needs to be restarted, any requests that have not been +answered will still appear in the file and will be read by the new +instance of the helper. + +Each cache should define a "cache_parse" method which takes a message +written from user-space and processes it. It should return an error +(which propagates back to the write syscall) or 0. + +Each cache should also define a "cache_request" method which +takes a cache item and encodes a request into the buffer +provided. + +.. note:: + If a cache has no active readers on the channel, and has had not + active readers for more than 60 seconds, further requests will not be + added to the channel but instead all lookups that do not find a valid + entry will fail. This is partly for backward compatibility: The + previous nfs exports table was deemed to be authoritative and a + failed lookup meant a definite 'no'. + +request/response format +----------------------- + +While each cache is free to use its own format for requests +and responses over channel, the following is recommended as +appropriate and support routines are available to help: +Each request or response record should be printable ASCII +with precisely one newline character which should be at the end. +Fields within the record should be separated by spaces, normally one. +If spaces, newlines, or nul characters are needed in a field they +much be quoted. two mechanisms are available: + +- If a field begins '\x' then it must contain an even number of + hex digits, and pairs of these digits provide the bytes in the + field. +- otherwise a \ in the field must be followed by 3 octal digits + which give the code for a byte. Other characters are treated + as them selves. At the very least, space, newline, nul, and + '\' must be quoted in this way. |